Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads mostly radiate, the rays pistillate and fertile, broad, yellow, mostly 5–35; invol bracts permanently erect, partly or wholly herbaceous, in 2–3 similar or sharply differentiated series; receptacle naked, hemispheric to merely convex; disk-fls yellow, perfect and fertile; style-branches flattened, with introrsely marginal stigmatic lines, truncate, minutely penicillate; achenes turbinate, hairy, mostly 5-angled; pappus of a few (typically 5) hyaline, often awned scales; aromatic herbs with alternate or all basal, entire to ternate or pinnatifid, commonly punctate lvs. 20, New World.